Inductors are often used in environments in which they are surrounded by other circuitry. A problem with such inductors is that the magnetic fields they radiate may interfere with the operation of the surrounding circuitry. It is therefore desirable to configure an inductor to maximise the cancellation of its magnetic field components at distance. Other elements of circuitry positioned in suitable orientations at distance from the inductor would consequently suffer minimal interference as a result of the magnetic field radiated by the inductor.
One configuration of an inductor that has been designed to address this problem is in the shape of a figure-of-8. WO 2004/012213 describes such an inductor, shown schematically in FIG. 1. Current enters the inductor via a feed line 101 that runs outside the boundary of the figure-of-8 shape from its base to its upper loop. The current flows around the structure in the directions indicated by the arrows. The current exits the inductor via a feed line 102 which runs outside the boundary of the figure-of-8 shape from its lower loop to its base. As a result of the crossover section in the middle of the figure-of-8 structure, the current flows clockwise around the lower loop and anticlockwise around the upper loop. As indicated using conventional notation on FIG. 1, the magnetic field created by the current flowing clockwise around the lower loop is directed into the page and the magnetic field created by the current flowing anticlockwise around the upper loop is directed out of the page. The field lines join such that most of the magnetic field components in the plane of the inductor are contained within the area of the figure-of-8 structure. A degree of cancellation of the magnetic field components is therefore achieved at distance from the inductor in the plane of the inductor.
Total cancellation of the magnetic field components is theoretically possible along an axis which bisects a figure-of-8 inductor structure such that the lower loop is on one side of the axis and the upper loop is on the other side of the axis. Total cancellation is achieved when the size and shape of the two loops are identical and perfectly symmetrical about the axis. The magnetic field components radiated from the two loops would be equal in magnitude but opposite in direction. For such a structure, small residual magnetic field components would remain at distance from the structure everywhere except along the axis defined above.
The inductor structure of FIG. 1 is not symmetrical about any axis defined in the plane of the inductor. The cancellation is further compromised in the design of FIG. 1 because the feed lines 101, 102 contribute to the magnetic field radiated by the lower loop. Partial compensation of this effect is achieved by reducing the area enclosed by the lower loop with respect to the area enclosed by the upper loop. Despite this, significant magnetic field components exist at distance from the inductor in all directions.
U.S. Pat. No. 7,151,430 describes other figure-of-8 inductor designs, one of which is shown schematically in FIG. 2. Current enters the inductor via a feed line 201 at the base of the lower loop. The current flows around the figure-of-8 structure in the directions indicated by the arrows and exits the inductor via a feed line 202 also attached to the base of the lower loop. As in FIG. 1 the current flows clockwise around the lower loop and anticlockwise around the upper loop. The inductor structure of FIG. 2 is symmetrical about an axis 203 which bisects the figure-of-8 structure such that half of the lower loop and half of the upper loop are on one side of the axis and the other half of the lower loop and the other half of the upper loop are on the other side of the axis. As a result of the symmetry exhibited by the structure, the magnetic fields radiated by the two loops are better matched in some respects than in the inductor structure of FIG. 1.
The structure of FIG. 2 is not symmetrical about a second axis 204. This is because the feed lines 201, 202 are connected to two ends of the lower loop. The magnetic field radiated by the lower loop is skewed compared to the magnetic field radiated by the upper loop because of the close coupling between the two loops in the crossover section. Consequently, cancellation of the magnetic field components at distance from the inductor is compromised. Therefore a resultant magnetic field remains at distance from the inductor in all directions. This resultant magnetic field is capable of causing significant disturbance to the surrounding circuitry.
There is thus a need for an improved inductor design which reduces the resultant magnetic field at distance from the inductor.